Gas filtering method using fabric filter

ABSTRACT

A method for impregnating a bundle of sized glass fibers with a composition comprising a polytetrafluoroethylene resin, a film-forming acrylic resin, a thickening agent and a polyhydrolyzable organosilane having nonhydrolyzable amine functionality is described. The impregnated bundles are especially suited for at least the warp of fabrics for filtering entrained particulates from gases such as flue gases from the combustion of hydrocarbon fuels such as coal.

This is a division of application Ser. No. 368,074, filed Apr. 14, 1982U.S. Pat. No. 4,450,197.

TECHNICAL FIELD

This invention relates to coated glass fiber fabrics, to methods andcoatings used in making fiber bundles for such fabrics and to methods ofusing such fibers to filter entrained particulates from gases,especially hot flue gases from the combustion of hydrocarbon fuels suchas coal. More particularly, this invention pertains to impregnatingbundles of sized glass fibers with compositions containingpolytetrafluoroethylene resin.

BACKGROUND ART

Heretofore protective coatings containing polytetrafluoroethylene resinhave usually been applied to glass fibers or glass fiber fabrics wherethe glass fibers have not been sized or have had any sizing removed(ordinarily by heating to about 1100° to 1300° F. after weaving intofabric) or have been pre-coated only with a glass-resin coupling agentsuch as an organosilane.

Thus, Fluatt, Wong and Walsh have disclosed in their applicationentitled POLYTETRAFLUOROETHYLENE FLUOROCARBON RESIN-CONTAINING COATINGCOMPOSITION FOR GLASS FIBERS, Ser. No. 200,675, filed Oct. 27, 1980, thedirect coating of virgin glass fibers with compositions containingpolytetrafluoroethylene resin which are similar to those employed in thepresent invention for impregnating bundles of sized glass fibers.

U.S. Pat. No. 3,838,082 to Sauer discloses the use of aqueousdispersions consisting essentially of film-formingpolytetrafluoroethylene and a polyhydrolyzable organosilane (includingamine-functional silanes) or organic titanate, zirconate or tincompound, together with an anionic or nonionic surfactant and,optionally, a water-repellent such as a fluorinated acrylate polymer,for coating desized glass fiber fabrics. Sauer discloses the alternativeuse of the same compositions for coating "sized glass fabric," i.e.,"griege goods." Sauer does not suggest the inclusion in his coatingcompositions of a second film-forming polymer or resin, other than thefluorinated acrylic polymers as optional water-repellents, and hepresents data which indicates that other properties are sacrificed whensuch fluorinated acrylic water repellents are included.

DISCLOSURE OF THE INVENTION

In one of its aspects, the invention comprises a method for impregnatinga bundle of sized glass fibers which method comprises (a) providing anessentially continuous bundle of glass fibers individually coated with asize comprising starch and a lubricant; (b) impregnating said bundlewith an aqueous impregnating composition comprising (i) emulsifyiedpolytetrafluoroethylene resin, (ii) emulsified acrylic resincoalesceable to form a coherent film upon drying an aqueous dispersionthereof at temperatures as low as about 10° C., (iii) a thickening agentcomprising base-solubilized polyacrylic acid, and (iv) apolyhydrolyzable organosilane having non-hydrolyzable aminefunctionality and/or hydrolysis and/or condensation products thereof;(c) drying the bundle containing said impregnating composition todeposit the non-volatile residue thereof in the interstices and on theouter surface of said bundle in contact with the sized glass fiberscomprising said bundle.

In other aspects, the invention comprises the impregnated bundles ofglass fibers so produced and fabrics woven with such bundles as at leastthe warp thereof.

In still a further aspect, the invention is a method for removingentrained particulates from a gas, such as a hot flue gas from thecombustion of coal or other fuel, by passing the gas through such afabric to filter out particulates.

BEST MODE FOR CARRYING OUT THE INVENTION

The method of the invention for impregnating bundles of sized glassfibers can employ as input any suitable glass fibers sized with anysuitable size compositions. Glass fibers of conventional commercialcomposition such as E-glass fibers are satisfactory for most purposes,although many special glasses known to the art may be employed wheretheir higher cost is thought to be justified by their unique properties.

Conventional sizing compositions for glass fibers can be applied to theindividual fibers, ordinarily as they are formed and before beinggathered into bundles, by any of the techniques known to the art, suchas roll coating, spraying, padding, etc. Suitable sizing compositionsinclude aqueous compositions comprising starch and lubricant, such as anoil or wax, many of which are known to the glass fiber art. Especiallysuitable are such compositions further comprising an acrylic resin,particularly one having a minimum film-forming temperature not higherthan about 10° C. The essentially continuous glass fibers to which thesizing composition has been applied will ordinarily then be gatheredinto one or more bundles, each bundle collected into a suitable package,such as by winding onto a sleeve or core, and then dried by heated airor other suitable means, although all or a portion of the drying can beaccomplished before collecting if desired. Bundles with very low or evenno twist are preferred.

The bundles of sized glass fibers can be impregnated with theimpregnating compositions of the invention by any suitable method. Onepreferred method is by immersing the bundle in a body of the aqueousimpregnating composition, as by pulling it around one or more rolls orbars under the surface of a body of the aqueous impregnatingcomposition, to wet the bundle with the impregnating composition, andthen drawing the bundle so wetted through a stripper die of suitablychosen dimensions to remove excess impregnating composition, which willordinarily be recycled to the impregnating bath, and also to aid inworking the impregnating composition throughout the bundle. While theamount of impregnating composition left in the bundle can vary widely,depending on both the concentration of solids in the impregnatingcomposition and on the subsequent processing and use contemplated forthe impregnated bundle, an amount corresponding to about 3 to about 10percent, and more preferably about 8 to about 10 percent, dried residueof the impregnating composition is ordinarily preferable, especially foruse in fabrics for filtering particulates from gas such as flue gas fromthe combustion of coal or other fuels.

The concentrations of the various components in the aqueous impregnatingcompositions of the invention can vary widely, but preferably will befrom about 3 to about 10 percent by weight of the emulsifiedpolytetrafluoroethylene resin, from about 1 to about 5 percent by weightof the emulsified acrylic resin, from about 0.1 to about 1.0 percent byweight of the thickening agent and from about 0.1 to about 1.0 percentby weight of the organosilane. Total solids will preferably be fromabout 5 to about 25 percent by weight.

The emulsified polytetrafluoroethylene resin can be any suitablepolytetrafluoroethylene resin which will not coalesce upon drying atordinary ambient temperatures to form a coherent film. Preferably thepolytetrafluoroethylene resin will also not coalesce to form a coherentfilm at the temperatures encountered in drying the aqueous compositionin the bundle of glass fibers. Preferably the polytetrafluoroethyleneresin will be emulsified as small particles from about 0.1 to about 1micron, and more preferably from about 0.1 to 0.2 micron, in averagediameter. Teflon (Reg. T.M. of DuPont) T-3338, a nonionic aqueousemulsion containing about 60 percent by weight polytetrafluoroethyleneresin and about 6.5 percent by weight emulsifying agent, and Fluon (T.M.of ICI) 583 are particularly suitable materials.

The emulsified acrylic resin can be any suitable acrylic resin whichwill form, with the other non-volatile components of the aqueouscomposition, a cohesive film adherent to the sized glass fibers.Polymers of lower alkyl esters of acrylic, and less advantageouslymethacrylic, acids are preferred. It is preferred that the minimumfilm-forming temperature of the emulsified acrylic resin be lower thanordinary ambient temperatures, and more preferably not higher than about5° C. A particularly suitable material is Rhoplex (Reg. T.M. of Rohm andHaas Co.) TR-520, which is a 50.5 percent solids anionic aqueousemulsion of an acrylic ester polymer having a minimum film formationtemperation of about 5° C.

The thickening agent can be any suitable water soluble polymer, of whichmany are available and known in the art, preferably an acid functionalacrylic polymer such as polyacrylic acid. A particularly suitablematerial is Acrysol (Reg. T.M. of Rohm and Haas Co.) ASE-108, abase-solubilized polyacrylic acid.

The organosilane can be any suitable polyhydrolyzable silane havingnon-hydrolyzable amine functionality. Among suitable materials which arereadily available are:

NH₂ (CH₂)₃ Si(OCH₂ CH₃)₃,

NH₂ (CH₂)₂ NH(CH₂)₃ Si(OCH₃)₃, and

CH₃ OC(═O)(CH₂)₂ NH(CH₂)₂ NH(CH₂)₃ Si(OCH₃)₃.

While these organosilanes are ordinarily considered to act asresin-glass coupling agents at resin-glass interfaces, we have foundthat their inclusion in the impregnating compositions of the inventionimproves the properties and performance of the products even though theimpregnating composition is applied to glass fibers already sized withan organic composition.

Additional conventional components may, if desired, also be included inthe aqueous impregnating compositions for use with bundles of sizedglass fibers according to the method of this invention. Among suchoptional components are fiber lubricants such as emulsifiable oils, ofwhich Quaker (T.M. of Quaker Chemical Co.) 1618-335, an emulsifiablesoybean oil, is a particularly suitable example, preferably at fromabout 0.1 to about 2 percent by weight, and defoamers.

EXAMPLE

A particularly suitable aqueous impregnating composition of theinvention is given by the following formulation.

    ______________________________________                                                            Weight Percent                                            Ingredient            As rec'd.                                                                              Solids                                         ______________________________________                                        TE-3338 (66.5%)       10.4     6.92                                           TR-520 (50.5%)        4.0      2.02                                           NH.sub.2 (CH.sub.2).sub.3 Si(OCH.sub.2 CH.sub.3).sub.3 (100%)                                       0.8      0.80                                           1618-335 (100%)       0.5      0.50                                           NH.sub.4 OH           0.5      --                                             ASE-108 (20%)         2.4      0.48                                           Nopco NDW defoamer (80%)                                                                            0.1      0.08                                           Water                 81.3     --                                             Total                 100.0%   10.8%                                          ______________________________________                                    

The impregnated bundles of glass fibers of the invention can be used asboth warp and fill, the latter preferably bulked or texturized forweaving into fabric or as only warp, substituting other fill. Othersuitable fill for use in fabrics of the invention, especially those tobe used for filtering particulates from gas such as flue gases from thecombustion of coal or other fuels, can be produced from bundles of sizedglass fibers, such as described hereinabove, by coating or impregnatingthem with an aqueous composition similar to that employed for the warp,but omitting the silane, during bulking or texturizing of the fillbundle.

If desired, fabrics of the invention can be coated after weaving withadditional polytetrafluoroethylene resin-containing compositions. Thiscan be accomplished by conventional fabric coating techniques using anaqueous dispersion of the polytetrafluoroethylene resin, such as duPontTE-3338 diluted with water to about 30% solids by weight followed bydrying. The total amount of all organic or volatile material in or onthe final dried fabric can vary considerably, but will preferably befrom about 8 to about 12, and most preferably about 10, percent byweight as determined by loss on ignition, of which the size willordinarily contribute not more than about 0.5 percent.

INDUSTRIAL APPLICABILITY

Since desizing glass fibers ordinarily results in significant impairmentof their physical properties, and since such impairment is usually moredetrimental in the warp than in the fill of woven fabrics, the provisionby the present invention of a method for impregnating bundles ofconventionally sized glass fibers with polytetrafluoroethyleneresin-containing compositions represents an important advance in thetechnology of providing glass fiber fabrics suitable for filteringparticulates from gases, especially hot flue gases, and for other uses.

Numerous modifications and variations of the invention as described willbe apparent to those skilled in the art, and such are contemplated aswithin the scope of the invention as claimed.

We claim:
 1. A method for removing entrained particles from a gas whichcomprises passing gas through a glass fiber fabric, wherein said fabricglass fibers have been sized with a size comprising starch and alubricant, then impregnated with an aqueous solution comprising:(a)emulsified polytetrafluoroethylene resin; (b) emulsified acrylic resincoalesceable to form a coherent film upon drying an aqueous dispersionthereof, said resin having a minimum film-forming temperature notgreater than about 10° C.; (c) a thickening agent comprisingbase-solubilized polyacrylic acid and, (d) a polyhydrolyzableorganosilane having non-hydrolyzable amine functionality, or hydrolysiscondensation products of said organosilane, or a combination thereof;said glass fibers having been dried to deposit the non-volatile residuethereof in the interstices and on the outer surface of said glassfibers.
 2. The method of claim 1 in which the temperature of said gas isnot greater than 600° F.
 3. The method of claim 2 in which said gascomprises flue gas from the combustion of hydrocarbon fuel.